In this study, response spectra of long-period earthquake ground motion (approximately from 5 to 15 seconds) at several cities in northern part of Japan due to Nihonkai-chubu Earthquake 1983 are estimated from sloshing height of oil storage tanks and recorded data of acceleration and displacement type strong motion seismographs. Especially in Niigata city which is about 270km away from the epicenter, very high sloshing is observed, from which equivalent velocity response spectrum is predicted to be more than 200 kine for 10 seconds structures with 0.1% damping. Mended and corrected displacement and acceleration. type strong motion seismograms are found to give much higher response spectra than design values for long period (5-10 seconds) structures with 2-5% damping.